dirty (clogged) pilot jets. happens alot, especially if the bike sat for any length of time unused. a 250 has REAL small pilot jets, so it doesn't take much at all to clog them up.

could also have water in the float bowls depending on how hard it was rained on. there's a drain screw on each carb bottom. turn the gas off, loosen the screws to drain the carbs, then tighten them back up, turn the gas back on, and see how it runs after that. if there's no difference, it's clogged pilots.

you can also try catching the gas as it comes out of the carbs. water is heavier than gas, and they don't mix, so you'll be able to see any water if it's in there.